HR: 08:30h
AN: T11G-03 INVITED     [Abstracts]
TI: Spreading Geometry and Melt Supply at the Ultraslow-Spreading Southwest Indian Ridge
AU: * Cannat, M
EM: cannat@ipgp.jussieu.fr
AF: IPGP-Geosciences Marines, 4 pl Jussieu, Paris, 75252 France
AU: Sauter, D
EM: daniel.sauter@eost.u-strasb.fr
AF: EOST-CNRS, 5 rue Rene Descartes, Strasbourg, 67084 France
AU: Mendel, V
EM: veronique.mendel@eost.u-strasbg.fr
AF: EOST-CNRS, 5 rue Rene Descartes, Strasbourg, 67084 France
AU: Ruellan, E
EM: ruellan@unice.fr
AF: CNRS-Geosciences Azur, 250 rue Albert Einstein Sophia Antipolis, Valbonne, 06560 France
AU: Okino, K
EM: okino@ori.u-tokyo.ac.jp
AF: ORI-Tokyo University, 1-15-1 Minamidai Nakano, Tokyo, 164-8639 Japan
AU: Humler, E
EM: humler@ipgp.jussieu.fr
AF: IPGP-Geosciences Marines, 4 pl Jussieu, Paris, 75252 France
AU: Mevel, C
EM: mevel@ipgp.jussieu.fr
AF: IPGP-Geosciences Marines, 4 pl Jussieu, Paris, 75252 France
AB: The Southwest Indian Ridge (SWIR) stretches 7900 km and is among the world's slowest spreading ridges with a full rate of only 1.4-1.5cm/yr. Regional axial depths vary between 3100m near Marion Island, and 4800m east of the Melville transform. Basalt Na8.0 contents increase correlatively and are consistent with variations of the ridge's melt supply corresponding with a magmatic crustal thickness of a little more than 6 km near Marion, to about 3 km east of the Melville transform. This along-axis change in melt supply is not correlated with ridge obliquity as would be the case for a homogeneous mantle rising passively beneath the ridge (upwelling velocities equal to effective half spreading rates). This shows that melt supply along the SWIR is not a simple function of spreading geometry. Instead, mantle temperature and/or mantle chemistry appear to be the principal controls on melt supply at the regional scale. We further investigate this issue at the smaller scale of the low melt supply end-member, easternmost portion of the SWIR between 61\deg and 67\deg E. We use the most extensive set of off-axis bathymetry, gravity and magnetic data available to date for an ultraslow spreading environment. We find that spreading rate and ridge geometry have changed little over the past 26myrs, with an average rate of 14.5cm/yr, and two broad sub-regions in terms of ridge obliquity: 35\deg in a 400km-long western region, and 7\deg in a 250km-long eastern region. We find no evidence for a systematic change in melt supply between these two sub-regions. Instead, we find evidence in both sub-regions for transient melt focusing events, and for longer-lasting periods of tectonically-dominated spreading, with large-offset normal faults and lower melt supply. We use available on-axis basalt chemistry data to propose that such short-lived and apparently randomly distributed pulses in axial melt delivery could be related to short-scale heterogeneities in the composition of the sub-axial mantle.
DE: 8434 Magma migration
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3655 Major element composition
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology and bottom photography
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting